Optical fiber consists of transparent material such as glass or plastic. Most optical fiber is fused silica and most plastic fiber is polymethylmethacrylate (PMMA). All optical fiber consists of a core and cladding of which the core has higher refractive index than the cladding. The fiber structure guides light by the process of total internal reflection (TIR). In silica fibers the core is usually established through doping with Germanium. PMMA uses a Fluorine polymer coating as the cladding. Fibers fall into two basic types, single mode or multimode. In single-mode fibers the core is very small, 5 to 10 microns in diameter, for instance. Multimode fibers have cores of 50 to several thousand microns and very small cladding (in the order of tens of microns). Single-mode fibers have a large cladding (usually more than 50 microns) making the fiber diameter generally 125 microns (FIG. 1). The purpose of the large cladding in single-mode fibers is to protect and contain the evanescent field of the single-mode which extends into the cladding for a few microns and can contain more than 10 percent of the optical energy normally thought of as traveling only through the core. Another importance of this larger diameter cladding is so that the fibers can be handled without breaking. However, with regard to fiber optics used for chemical sensing, the cladding must be permeable to the substance being sensed. Thus, a small diameter fiber surrounded by a thick cladding is not practical for chemical fiber optic sensors.
Large core fibers experience a relatively large number of spatial modes, possibly in the hundreds. Light traveling in different spatial modes travels at different speeds. Due to unavoidable perturbations, light can and does couple from one mode to another (so-called “mode mixing”). Mode mixing, and different light speeds between various modes causes noise and uncertainty in light detection systems and causes pulse spreading in communication systems. For this reason, single-spatial mode (single mode) fibers are used in many communications and sensing systems. One advantage, however, of multi-mode fiber is its large core area has a diameter with sufficient structural integrity and can be coated with a chemically sensitive polymer which can function as its cladding. In fact, chemically sensitive multi-mode fibers have been realized by embedding chemical indicators in a porous polymer coating which functions as the fiber cladding. Light traveling along the fiber core with its evanescent field extending into the cladding will experience higher loss when the indicator is triggered by the presence of a specific gaseous chemical.
Because the core size of a standard single-mode fiber is typically 10 μm, it is too fragile when cladded with porous indicator-embedded polymers that have been proven for use on multimode fibers. This is because the glass is thin, and polymer is not sufficiently rigid to maintain structural integrity of the fiber. FIG. 2 shows an optical fiber for use in chemical sensing that includes a glass core and a cladding material. Although the optical fiber of FIG. 2 is durable, it is a multi-mode fiber
Resonators have been implemented in chemical sensors to circulate light around an optical fiber loop for multiple passes. A periodic series of resonance lineshapes is produced, each having a peak centered about a resonance frequency under normal conditions, and the resonance lineshape has a finesse associated therewith. The frequency-periodicity of frequency separation between resonance frequencies of the same mode is the free spectral range of the resonator. As used herein, the term “finesse” refers to a relationship (e.g., sharpness) based on a ratio of the free-spectral range to the linewidth of an individual resonance lineshape. The linewidth of the resonance lineshape is a frequency width at half of the maximum peak value of the resonance lineshape. The finesse additionally relates to the number of times the light recirculates within the optical loop with reproducibility, and thus is inherently related to the round-trip loss of the resonator. Higher losses generally result in lower finesses. It is generally difficult to couple light into a multi-mode optical fiber and maintain the light in a single spatial mode that reproduces itself for multiple circulations through the resonator. For example, perturbations (e.g., imperfections, geometrical distortions, etc.) along the length of the optical fiber typically decrease the round-trip reproducibility of the single fiber spatial mode within a multi-mode fiber, and thus decrease the finesse. Other spatial mode resonances can also be excited which typically cause errors in the intended measurement. In the latter case, a complex structure of resonances, which may be based on a single stable resonance, may be observed that create instabilities and errors in the measurement. Each spatial mode may be associated with two polarization modes, which doubles the number of resonances in the spectrum.
A single mode optical fiber may be used to significantly improve the resonance characteristics of the resonator by assuring that a single spatial mode of the fiber supports the resonance mode of the resonator. For example, this single spatial mode is the sole resonating mode provided that one polarization state is resonating within the resonator. Instabilities created by power sharing between several spatial modes of the fiber and errors resulting from the presence of several resonator modes are thus substantially eliminated. Measurements of the finesse, the linewidth of the resonance, and the free spectral range are typically unique since these relate to the loss and pathlength for light traveling within a single spatial mode of the fiber and for a single resonance lineshape. To make a chemically sensitive fiber, the light should interact with the polymer. Placing a permeable, chemically sensitive polymer cladding directly on the core of a typical single mode fiber is generally impractical because the core is too small, as discussed above. Adding an intermediate glass cladding between the core and a polymer coating would tend to interfere with sensing.